The present invention relates to a method of producing rubber mixtures in two stages, namely a master-batching stage and a final mixing stage, with the maximum temperature during final mixing being less than the maximum temperature during master batching. The method may also be a discontinuous method that uses kneaders, with material that is to be mixed passing successively through the kneaders. The present invention also relates to an apparatus for carrying out the aforementioned methods.
Of particular significance during the manufacture of rubber articles is the production of vulcanizable rubber mixtures from the necessary components, namely rubber, fillers, and other additives, and from the components that are necessary for vulcanization or bonding. Next to the units for vulcanization, the units needed for mixing and preparation represent the greatest investment in a rubber plant. Internal mixers are primarily used to manufacture such mixtures.
The mixing process performs two different tasks:
(a) on the one hand, the additives that are necessary in order to provide the preparation and usage properties, such as, for example, highly active and other fillers, plasticizers, processing aids, anti-oxidants, ozone protectors, pigments, etc., must be dispersed as rapidly and as effectively as possible. In this connection, depending upon the mixing unit that is used, the type of mixture, and the mixing conditions, temperatures of up to 150.degree. C. and even greater can be achieved during master batching without damaging the mixture; and
(b) on the other hand, however, the materials that are necessary for polymerization and bonding to substrates, such as, for example, sulfur and vulcanization accelerators, peroxides, vulcanization resins, RF-systems, etc., must be added in at such low temperatures that in so doing no premature reactions occur. With moist accelerator and bonding systems, the temperatures may not exceed 100.degree. to 110.degree. C. during final mixing.
Where larger quantities have to be dealt with, especially in the tire industry, a two-stage process is therefore used for the mixing. First of all, the additives are added to the rubber at relatively high temperatures; this is the master batching. The mixture is then cooled in a recovery unit and subsequently in air or water, and is finally intermediately stored. Then, in a second stage, the materials that are necessary for polymerization and bonding to substrates are added at a temperature that is lower than the master batch temperature; this is the final mixing. During such final mixing, the temperature must be low enough that the rubber does not already begin to polymerize in the mixer.
The manner in which the heretofore known mixing process carries out these two stages entails expensive transportation within the plant area, increases the occurrence of unusable residual quantities, and requires a dosing of the respective mixture components prior to both stages of the process.
In order to avoid these drawbacks, so-called single-stage kneading processes have been developed where the two decisive stages of the process, namely the master batching and the final mixing, accompanied by the interposition of a cooling process, are effected one after the other within a single unit. However, kneaders that operate in this fashion work too slowly. For the large quantities that are required to be dealt with in tire plants, several of these kneaders must be installed parallel to one another. Thus, these kneaders would require an investment that is too great, and would require too much space in a plant.
It is therefore an object of the present invention to shorten the overall mixing time, and to simplify the mixing apparatus.